We will see if ABCs have encountered antigen and undergone somatic hypermutation and class switching. Diversity in the repertoire and the mutation frequency will be examined in old mice. This may determine if the old cells are exhausted from chronic antigen exposure. To determine what antigens induce non-responsive B cells, we will sequence heavy and light chains from single cells, clone them into expression vectors, and test for reactivity to a panel of antigens. To see if ABCs prevent affinity maturation of follicular cells, we will transfer cells into recipient mice, immunize, and measure affinity.